Blunting Excalibur’s edge

Why are some US-supplied weapons relying on the Global Positioning System falling prey to Russian jamming?

May 2024 finished with bad news. Some of the weapons the United States had supplied to Ukraine were losing their effectiveness. Systems such as the BAE Systems/Raytheon M982 Excalibur 155 mm artillery shell and Lockheed Martin’s M142 HIMARS (High Mobility Artillery Rocket System) experienced degradations in their accuracy. Both weapons use position, navigation and timing (PNT) signals transmitted by the US Global Positioning System (GPS), a global navigation satellite system (GNSS) to improve their precision. Russian radio frequency (RF) jamming appears to have been successful in jamming these PNT signals. As a result, the scalpel-sharpness of these munitions has been significantly blunted. This revelation was not the rambling lie of some Russian troll factory. Rather, reputable media outlets such as the Washington Post ran the story which was based on confidential Ukrainian documents seen by that paper’s journalists.

To understand Russian GPS jamming, it is necessary to understand how GPS works. The first issue is the weakness of PNT signals when they reach Earth. A rule of thumb in radio frequency engineering is that a radio signal loses strength the further it travels. GPS PNT radio signals are no exception. A PNT signal can have a strength as weak as -127 decibels (dB) by the time it travels the 20,200 km from its satellite in space. This decibel figure is lower than the normal electromagnetic background noise. It is the unique coding structure of the GPS signal, which effectively boosts the incoming signal power by about 23 dB. This enhancement allows the incoming signal to be found against that noise in the first place.  Once a receiver finds the signal, that advantage increases by another 20 db, ensuring that it can be tracked well under normal conditions. Nonetheless, even with those gains, the inherent weakness of the signal makes it relatively easy to jam. All that is needed is a much stronger signal to be transmitted into a GNSS receiver within range of the jammer to ‘wash out’ the PNT signal. The R-330ZH Zhitel is a vehicle-mounted GNSS jammer used by the Russian Army for tactical and operational GNSS jamming. Zhitel can generate a jamming signal with a strength of between -53 dB and -56 dB at a range of 30 km from the GNSS receiver. The stronger signal of the Zhitel can risk washing out the weaker PNT transmission from space.

The GPS satellite constellation transmits several PNT signals. The main signals known as L1 and L2. L1 uses a frequency of 1.57542 GHz and L2 employs a frequency of 1.22760 GHz. Although originally designed for use by the US, and later Allied militaries, civilians have used GPS PNT signals since 1996. That year, US President Bill Clinton issued a policy directive to that end. The US military performed its first wartime use of GPS five years earlier when the system supported Operation Desert Storm which was launched in 1991. The war to liberate Kuwait from Iraqi occupation brought precision-guided weapons into the public consciousness. GPS was touted as a key technology which helped the US military navigation in the wilds of the Arabian Desert. Even though most weapons used in the conflict were not GPS-guided, their use presaged the impact such weapons would have in future conflict. The availability of GPS PNT signals to civilians has since revolutionised our world. Everything from financial transactions to public transport networks and even food deliveries can depend upon GNSS technology. GPS has since been joined by counterparts such as the European Union’s Galileo, the People’s Republic of China’s Beidou, and Russia’s GLONASS constellations.

However, every military advantage will eventually yield a vulnerability as countermeasures are developed to neutralise the tactical or operational benefits an innovation brings. The US was once again leading a coalition of nations in the Arabian Desert in 2003 with Operation Iraqi Freedom (OIF). On this occasion, the objective was not to liberate Kuwait, but to remove Iraqi dictator Saddam Hussein from power. The eve of the war saw press reports that the Iraqi military had purchased jammers capable of attacking the GPS constellation’s signals. Clearly, Saddam Hussein’s regime was determined to protect potential targets in Iraq from GPS-guided weapons. The Iraqi ruler was also no doubt mindful of the revolution in precision-guided munitions (PGMs) that had occurred since Desert Storm. The US government’s own figures say that 9% of all weapons used during that campaign were PGMs. By the time of Operation Allied Force (OAF) in 1999, the share of PGMs in all air-launched ordnance had grown to 35%, many of which were GPS-guided.

OAF was carried out by NATO in the skies above Kosovo and Serbia, with the aim of stopping Serbia’s ethnic cleansing of Kosovo’s ethnic Albanian population. OAF saw the debut of a revolutionary PGM in the form of Boeing’s Joint Direct Attack Munition (JDAM) guidance kit. Comprising a nose and tail fin assembly, JDAM kits outfit an array of ‘dumb’ bombs. Using a combination of a GPS receiver and an inertial navigation system (INS), the JDAM assembly receives GPS PNT signals. These signals allow the weapon to determine its own location vis-à-vis its target. The tail fins continually adjust the course of the bomb during its flight to ensure it lands as close to the target as possible. Publicly available information claims that JDAM kits provide accuracies of up to 5 m circular error probable (CEP) when GPS and INS are used together. A JDAM-enhanced bomb can still reach its target if the GPS PNT signal is unavailable, but with degraded accuracy. Using the INS alone, the bomb will fall within 30 m of its target.

GPS Codes

As noted above, the GPS constellation sends its signals across two main links, chiefly L1 and L2. Newer satellites joining the GPS constellation transmit PNT signals on the L5 frequency of 1.17645 GHz. All of the GPS constellation’s PNT signals fall within the L-band (1–2 GHz). The employment of L-band was a deliberate choice by the GPS system’s designers. At the time, L-band had enough space to accommodate the relatively wideband 20 MHz GPS signals. Moreover, L-band signals tend not to get excessively degraded by weather. An omnidirectional antenna can also be used to receive L-band signals. What this means in practice is that it is unnecessary to physically point a GPS receiving antenna in the satellite’s direction to obtain the signal.

On their way to Earth, GPS signals must travel through the ionosphere – an ionised section of the atmosphere found at between 48 km and 965 km above the Earth’s surface. Atoms in the ionosphere can become excited by solar radiation and it is this excitement that can cause problems for some radio signals traveling from space to Earth. The ionosphere can also cause delays to some signals as they approach the planet, but these delays tend to be manageable for L-band signals. Ionospheric delay is the reason why multiple link frequencies are used. By using a GPS receiver which obtains signals from both the L1 and L2 links, the receiver can compare each signal and account for this ionospheric delay.

Since navigation depends on accurate timing, GPS satellites are equipped with atomic clocks which have a stable output frequency of 10.23 MHz. That means the atoms which comprise the radioactive materials of the satellite’s atomic clock resonate at a rate of 1,023,000 times per second. The frequencies used by GPS constellation’s signals are multiplications of this 10.23 MHz frequency. For example, the L2 frequency is the 10.23 MHz signal multiplied 120 times. The L1 signal is the 10.23 MHz signal multiplied 154 times.

GPS satellites transmit coded signals. The civilian standard coded signal is known as C/A for Coarse Acquisition, which is carried on the L1 link and is unencrypted. L1 also carries the military encrypted precision P-code and will also carry the military encrypted M-code in the future. In addition, L1 carries the new L1C civilian code. L1C is transmitted by the GPS Block-III constellation, the first spacecraft for which was launched in December 2018. The new signal uses a frequency of 1.57542 GHz. While there is not enough space here to fully explain the distinctions between the L1 and L1C signals, suffice to say that L1C enables better interoperability for GPS receivers vis-à-vis the European Union’s Galileo GNSS constellation.

The L2C signal has been available on all GPS satellites launched since 2005, and this comparatively new signal affords civilian GPS users the ability to correct for ionospheric delay, thereby providing them with improved accuracy, although very few civilian GPS receivers are using it. An additional signal known as L5, which is transmitted on a frequency of 1.17645 GHz, was made available from May 2010. The L5 signal is primarily used for safety-of-life applications such as emergency locator beacons and in civil aviation.

The code transmitted by these signals is comprised of multiple ‘bits’, basically ones and zeros, which are also known as ‘chips’. These bits are generated in a pseudorandom fashion meaning that they are generated by computer but are considered statistically random. Each sequence of bits is sent at a set rate of bits-per-second for a set length of time. The GPS receiver will generate a replica of GPS code at a specific moment and record the time while receiving the incoming code from the satellite. The receiver will calculate the time lag between each zero and one in the incoming code relative to each zero and one in the replica code. Let us suppose that this gap is one second. Like all radio transmissions, PNT signals travel at the speed of light (299,792 km/s in a vacuum), so a gap of one second equates to roughly 299,792 km. Thus, the GPS receiver is that distance from that specific satellite. To ascertain location, the receiver must obtain PNT signals from at least three other satellites. By repeating this process and triangulating the three bearings from each satellite, the receiver can determine its position.

A general rule of thumb regarding GPS is that the faster the bit-per-second rate of the signal, the more precise the GPS receiver will be in determining position. C/A-code has a sequence of 1,023 bits-per-second transmitted at a rate of 1,230,000 bits-per-second meaning the C/A-code’s sequence repeats each millisecond. P-code is transmitted on the L1 and L2 signals but a key difference between P-code and C/A-code is that the former is transmitted at a rate of 12,300,000 bits-per-second; ten times faster than the latter. The higher bit-per-second rate gives more precise time-lag measurement, translating into a more precise range measurement, and since the P-code is transmitted on both L1 and L2, it allows the time delay of the ionosphere to be removed.  For example, C/A-code may give about 4 m of accuracy, says Douglas Loverro, President of Loverro Consulting and former Deputy Assistant Secretary of Defence for space policy in the US Department of Defense (DOD). According to Loverro, who was a key architect of the GPS system, “The P-code lets you get down to under three metres (ten feet) of accuracy.”

P-code is secured using encryption to avoid it being spoofed or jammed with the encrypted signal known as P(Y)-code. “P(Y)-code was designed with encryption to avoid the signal being spoofed and to avoid other people using the P signal who should not be,” Loverro explained. Two things need to happen to allow a GPS receiver to use P(Y)-code. The moment a receiver is switched on, it must first acquire the C/A-code. Once this is done, it can then begin to receive the P(Y)-code. This can be a potential disadvantage if C/A-code signals are being jammed locally, Loverro noted. Any GPS receiver in range of the jamming will simply be unable to receive the C/A-code, let alone the P(Y)-code. It is noteworthy that some GPS receivers currently do have the ability to receive P(Y)-code without obtaining the C/A-code first. To use the encrypted P(Y) code, users need to have a decryption key they can load into their GPS receivers, Loverro added. The key decrypts the incoming P(Y)-code allowing the receiver to use that signal.

The US DOD provides the authorisation for users to receive the P(Y) keys and to load them into their devices. Given the jamming that the Ukrainian military has suffered when it has used GPS-guided weaponry, it is unclear if they were provided with a P(Y)-code capability in these weapons’ GPS receivers. This is perhaps not surprising. Access to P(Y)-code is reserved for the US’s NATO Allies on a case-by-case basis, according to Loverro. The DOD is understandably concerned about the security aspects of sharing P(Y) codes with Ukraine lest decryption keys fall into Russian hands.

M-Code

P(Y)-code shortcomings are being addressed with the advent of M-Code, which is yet to enter service, but could prove to be a potent enhancement. Firstly, M-Code is transmitted using L1 and L2 signals. The signal will be transmitted with more power to provide added resistance to jamming. It also does not need to connect with C/A-code before being used. M-Code also ‘plays nice’ with the existing C/A- and P(Y)-codes and, like P(Y)-code, M-Code is encrypted. The good news is that US and allied militaries are now embracing M-Code. The bad news is the Ukrainian military is unlikely to be gain access for similar reasons to the P(Y)-code quandary.

Nonetheless, Russian EW cadres may soon find out that switching on their GPS jammers maybe akin to signing their own death warrants. The Defence Post reported in May that Scientific Applications and Research Associates (SARA) received a contract worth USD23.6 million for a JDAM Home-On Jam (HOJ) capability. Specifically, this will be an augmentation to the GPS receivers used by the GBU-62 JDAM-ER (extended range) variant of the guidance kit. Details on how the HOJ capability will work are scant, but it is possible that the GPS receiver will be programmed to recognise an abnormal GPS signal. As noted above, GPS jamming typically relies on using a much stronger fake PNT signal to wash out the relatively weak true signal.

Should the receiver detect a jamming transmission, it will ascertain the signal’s bearing. The munition will guide itself along the bearing until it reaches the signal’s point of origin, announcing its arrival with a bang. This tactic is akin to that used against hostile ground-based air surveillance and fire control/ground-controlled interception radars by anti-radiation missiles. While Kyiv is unlikely to receive P(Y)-code keys, or even access to M-Code any time soon, Ukraine’s military has been supplied with JDAM-ER weapons. Reports regarding the HOJ capability say this will be made available to Ukraine, with the work being completed by October 2025. While this date appears some time away, it may be possible that upgraded JDAM-ERs will be drip-fed as built into the Ukrainian military. Looking further ahead, perhaps similar improvements may be added to HIMARS and Excalibur in the future. Either way, such improved weapons will give Russian EW cadres a Hobson’s choice: to switch on their jammers to protect their troops and assets against GPS-guided weapons? Or switch off their jammers and save themselves but possibly doom your comrades? How Russian forces will resolve this dilemma remains to be seen.

Thomas Withington